This invention relates to doped diamond and more particularly to doped diamond produced by chemical vapour deposition (hereinafter referred to as CVD diamond).
There are a range of applications of diamond for which a doped diamond layer of significant dimensions, with a uniform dopant concentration and associated electronic and/or optical properties would be advantageous. Dependent on the detailed application, this material needs to substantially exclude detrimental electronic or optically active traps or defects. To date, material of this type has not been available.
Applications such as high power electronics require bulk free standing diamond with thicknesses ranging from 50 to 1000 μm and lateral sizes varying from 1×1 mm2 to 50×50 mm2. For viable production in a competitive market it is beneficial that the diamond used for these structures is grown as a bulk material and processed into the final devices. In addition, wafer scale processing is possible with larger pieces, further reducing device fabrication costs. For optical applications, such as filters and absorbed power measurement devices, the large size and thickness of the raw material can be an intrinsic requirement of the device. Thus there are a range of benefits to synthesising thick layers.
Boron is the only known dopant in diamond which has well characterised relatively shallow dopant behaviour. Other potentially shallow dopants reported in the literature to be under investigation include S, P, O, Li, but these are not yet available as reliable bulk dopants. There are many electronic applications which need doped diamond, often over relatively large areas and with very uniform properties. However, the incorporation of boron during synthesis is a very sensitive property of the particular growth sector. Polycrystalline diamond contains a random selection of growth sectors, and although the average boron concentration may be uniform on a scale much larger than the grain size, at the same scale as the grain size the local boron concentration varies substantially from point to point.
Dopants can also be put into diamond by post growth treatment. The only currently reliable post growth treatment applicable to diamond is ion implantation, and this provides a method of producing layered diamond structures, but not uniform bulk doping. For instance, a ‘p-i’ (p-type—intrinsic) structure can be produced by using an appropriate dose and energy for boron implantation into a high quality natural type IIa diamond. Unfortunately residual damage (vacancies and interstitials) is always created under conditions of ion implantation. This damage is impossible to remove completely, although annealing treatments can reduce it. The damage leads to degraded charge carrier properties resulting from defect scattering and compensation of boron acceptors.
Methods of depositing or growing material such as diamond on a substrate by chemical vapour deposition (CVD) are now well established and have been described extensively in the patent and other literature. Where diamond is being deposited on a substrate by CVD, the method generally involves providing a gas mixture which, on dissociation, can provide hydrogen or a halogen (e.g. F,Cl) in atomic form and C or carbon-containing radicals and other reactive species, e.g. CHx, CFx wherein x can be 1 to 4. In addition, oxygen containing sources may be present, as may sources for nitrogen, and for boron. In many processes inert gases such as helium, neon or argon are also present. Thus, a typical source gas mixture will contain hydrocarbons CxHy wherein x and y can each be 1 to 10 or halocarbons CxHyHalz wherein x and z can each be 1 to 10 and y can be 0 to 10 and optionally one or more of the following: COx, wherein x can be 0.5 to 2, O2, H2 and an inert gas. Each gas may be present in its natural isotopic ratio, or the relative isotopic ratios may be artificially controlled; for example hydrogen may be present as deuterium or tritium, and carbon may be present as 12C or 13C. Dissociation of the source gas mixture is brought about by an energy source such as microwaves, RF (radio frequency) energy, a flame, a hot filament or jet based technique and the reactive gas species so produced are allowed to deposit onto a substrate and form diamond.
CVD diamond may be produced on a variety of substrates. Depending on the nature of the substrate and details of the process chemistry, polycrystalline or single crystal CVD diamond may be produced.
Obtaining incorporation of boron into the solid during deposition is less difficult than for many other potential dopants. The incorporation ratio for boron, which is the ratio of the dopant boron (B) to carbon (C) concentration in the solid ([B]/[C]:solid), compared to that in the depositing gas ([B]/[C]:gas) is generally about 1 (in the {100} growth sector) although it varies with many factors. There are many methods by which CVD diamond may be doped during synthesis with boron. With microwave plasma, hot filament and arc jet techniques, diborane (B2H6) or some other appropriate gas may be added to the gas stream, the incoming gases may be bubbled through methanol or acetone containing boria (B2O3), boron powder may be placed in the chamber, or a boron rod inserted into the plasma. For growth by the combustion flame method a fine mist of methanol containing boric acid can be injected into the gas stream with an atomiser. Diamond films have also been doped unintentionally when, for example, the plasma has decomposed a substrate holder fabricated from hexagonal boron nitride.
Nitrogen can also be introduced in the synthesis plasma in many forms. Typically these are N2, NH3, air and N2H4.
Although high purity single crystal (SC) CVD diamond has an important role in potential high power electronics, the number of potential applications would be substantially increased if a CVD doped diamond with uniform and advantageous electronic properties was available. In addition, there are other applications of boron doped diamond where uniformity in the colour, luminescence, or other properties associated with B doping is advantageous.